1. Field of the Invention
The present invention relates to an optical device and to a projection display incorporating such an optical device.
2. Description of the Related Art
GB 9 811 782.3 discloses a projection display in which a transmissive spatial light modulator (SLM) in the form of a liquid crystal device (LCD) is illuminated by an illumination source. The display is of the single panel type and includes a holographic field element and a projection optical system. Each picture element (pixel) of the SLM is associated with a holographic field element reflector which reflects light from the source to the entrance aperture or pupil of a projection lens. The field function of the display is thus incorporated in the holographic reflectors. The holographic reflectors may be patterned in order to generate a spatially multiplexed image without the use of absorptive colour filters.
This arrangement has the inconvenience that either the holographic field element reflector in the form of a volume hologram must be incorporated immediately adjacent the liquid crystal layer which involves manufacturing difficulties, or the holographic reflectors may be incorporated inside the counter substrate of the SLM, which reduces the aperture ration and hence the display brightness.
FIGS. 1 and 2 of the accompanying drawings illustrate a known type of single panel projection display as disclosed in H. Hamada et al, IDRC, 1994, pp 442-423 xe2x80x9cA new bright single panel LC-projection system without a mosaic colour filter xe2x80x9d and in U.S. Pat. No. 5,164,102. An illumination source (not shown) directs collimated white light to a set of relatively titled dichroic mirrors 1 for reflecting red, green and blue light with a relative angular separation. The resulting colour component beams are directed to a microlens array 2 disposed on the surface of a monochrome thin film transistor (TFT) LCD 3. Each microlens of the array 2 is disposed above three pixels and focuses the red, green and blue light beams onto the apertures of respective ones of the three pixels. The LCD 3 is of the transmissive type and the modulated light therefrom passes through a field lens 4 and a projection lens 5 such that the image is projected onto a screen 6.
The size of the image of the light source produced at each pixel is determined by the system etendue, the pitch of the LCD pixels and the thickness of the glass substrate. Etendue is a term representing brightness at any point in a system and is defined as the product of the beam area and solid angle of the beam divergence. For efficient coupling of optical radiation through a projection system, the etendue should be matched at each point through the system. Etendue mismatch causes a loss of brightness. In this transmission panel system, the light must be focused into the relatively small aperture of the pixel. Thus, the solid angle of the optical beam will be required to increase to compensate in order to maintain brightness. If the glass substrate is too thick then such a solid angle cannot be achieved by the microlenses and light is lost around the edge of the pixel. Also, green light may spill from the aperture of the green pixel into the red pixel and so on.
This type of arrangement works well for large low resolution LCD panels. However, if it is required to provide a higher resolution display for a given size of LCD panel, smaller pixel sizes must be provided and it may not be possible to illuminate such smaller pixels accurately with the available etendue. Alternatively, a larger panel may be used but this results in an increase in the bulk of the system.
C. Joubert et al, xe2x80x9cDispersive holographic microlens matrix for single LCD projectionxe2x80x9d, SPIE vol 2650 pp 243-249 discloses a projection display as illustrated in FIG. 3 of the accompanying drawings. An arc lamp 7 and parabolic reflector 8 supply collimated white light to a phase volume holograph 9 which separates the illuminating white light into red, green and blue beams with a small angular separation between each adjacent pair of beams. A cylindrical microlens array 10 focuses the R,G,B beams into the apertures of pixels in an LCD 11. This display is of the same type as that disclosed in Hamada but with the dichroic filters replaced by the hologram 9. Accordingly, this display suffers from the same problems associated with beam etendue, panel size and glass thickness.
JP 9 015 626 A discloses a reflection mode projection display as shown in FIG. 4 of the accompanying drawings. An LCD 12 is provided with a microlens array 13 having microlenses formed on its upper and lower surfaces. The LCD 12 comprises composite pixels, each of which is aligned with a respective microlens 14 formed on the front surface of the array 13. Each composite pixel comprises individual sub-pixels 15,16 and 17 aligned with respective microlenses such as 18 on the lower surface of the array 13. The sub-pixels 15,16 and 18 modulate red, green and blue light respectively, so as to provide a single panel colour display.
The LCD 12 incorporates plane reflectors, each of which forms part of a respective colour component sub-pixel. The green reflector 19 is disposed in the plane of the LCD 12 whereas the red and blue reflectors 20 and 21 are tilted in opposite directions.
Incident collimated colour component light beams are shown at 22B,22G and 22R. Each of the light beams is focused by the microlens array 13 to the aperture the corresponding sub-pixel and onto the corresponding reflector 19 to 21. The reflectors 19 to 21 have no optical power and reflect the colour component light beams substantially back along the incident light paths. Thus, light is reflected back through the same microlenses so that the input and output pupils are at the same location. It is therefore necessary to provide a beam splitter in the optical system so that the output light from the LCD 12 for projection can be separated from the path of the input light from the illumination system. This results in increased bulk and weight together with light and contrast losses. Also, as described hereinafter, vignetting occurs and this results in further light losses.
JP 10221681 also discloses the use of a microlens array on the front surface of a reflective LCD. Each colour component sub-pixel of he LCD has a plane reflector which gives rise to vignetting and loss of light. Also, the microlens array is of the two dimensional type and further loss occurs because of lens edge effects and the reduced aperture ratio of lenses. However, separate input and output pupils are provided so that a beam splitter is not required.
U.S. Pat. No. 5,825,443 discloses an arrangement which is similar to that disclosed in JP10221681 and which therefore suffers from the same disadvantages.
EP 0 953 865 discloses a projection display which includes an optical modulator comprising an LCD of reflective type and two microlens arrays. This microlens arrays are aligned with each other and have a pitch which is three times the pitch of the individual colour component pixels of the LCD. Each aligned pair of microlenses is said to constitute a relay optical system having a magnification of 1. The rear electrodes of the LCD pixels constitute plain reflectors. The use of the second microlens array between the first microlens array and the LCD results in increased chromatic aberration which reduces the coupling efficiency of the device. Also, there are several interfaces between materials of different refractive index resulting in substantial losses due to Fresnel reflection which reduces the efficiency of the device. Further, the separation between the second array of microlenses and the reflective pixels results in loss of brightness because of vignetting. Also, it is necessary during manufacture to align the two microlens arrays in he counter substrate and then align this to the back plain of the LCD, which makes the device difficult and expensive to manufacture.
According to a first aspect of the invention, here is provided an optical device comprising an array of convergent microlenses disposed in front of a reflective spatial light modulator comprising a plurality of rear reflectors, characterised in that each of the near reflectors has convergent optical power.
Each of the reflectors may be arranged to form a laterally displaced image of a microlens aperture or a part thereof substantially at the plane of the apertures of the microlenses of the array. The size of the image may be substantially the same as the size of the microlens aperture or part thereof. For a predetermined direction of incident illumination, each of the reflectors may be arranged to form an image of a first microlens aperture or part thereof at or within a second microlens aperture or part thereof.
Each of the reflectors may have a focal length substantially equal to half the optical path between an associated one of the microlenses and the reflector.
Each reflector may have convergent optical power in a first direction transverse to an optical axis of the device and may have no optical power in a second direction transverse to the optical axis of the device and substantially perpendicular to the first direction. Each reflector may be blazed in the second direction and the first and second microlens apertures may comprise different parts of the same microlens aperture.
Each reflector may comprise a metallised relief structure. Each reflector may be faceted.
Each reflector may comprise a volume holographic element.
Each of the microlenses may have optical power in a third direction transverse to an optical axis of the device and no optical power in a fourth direction transverse to the optical axis of the device and substantially perpendicular to the third direction. The third and fourth directions may be substantially parallel to the first and second directions, respectively. The array of microlenses may comprise a one dimensional array of cylindrically converging microlenses.
Each microlens or part thereof may be associated with a respective set of the reflectors. The spatial light modulator may comprise a plurality of composite picture elements, each of which is associated with a respective microlens or part thereof, and comprises a plurality of sub-picture elements, each of which is associated with a respective one of the respective set of the reflectors. The reflectors of each set may be different from each other and corresponding reflectors of the sets may be substantially identical to each other.
The spatial light modulator may comprise a liquid crystal device. The reflectors may be disposed between a liquid crystal layer and a substrate of the liquid crystal device. The reflectors may planarised. As an alternative, a substrate of the liquid crystal device may have a surface relief corresponding to the reflectors so that the liquid crystal layer has a substantially uniform thickness.
According to a second aspect of the invention, there is provided a projection display characterised by comprising a device accordingly to the first aspect of the invention, an illumination system for illuminating the device, and projection optics for projecting an image corresponding to modulation of light from the illumination system by the spatial light modulator.
The projection optics may have an input pupil which is laterally spaced from an output pupil of the illumination system. The output pupil may be disposed off-axis with respect to the device. The input pupil may be disposed on-axis or off-axis with respect to the device.
The display may comprise a field lens disposed between the illumination system and the array of microlenses.
The illumination system may comprise separating means for angularly separating red, green and blue components of light for illuminating the spatial light modulator. The separating means may comprise a plurality of relatively tilted dichroic mirrors. As an alternative, the separating means may comprise a diffraction element, which may comprise a blazed diffraction grating.
The illumination system may be arranged to illuminate the spatial light modulator with light having a first polarisation and the projection optics may comprise a linear polariser for passing light from the spatial light modulator having a second polarisation substantially orthogonal to the first polarisation.
The device may comprise a linear polariser. The device may comprise a patterned half wave retarder comprising first regions whose optic axes are parallel or perpendicular to the transmission axis of the polariser and second regions whose optical axes are oriented at 45xc2x0 to the optical axes of the first regions. As an alternative, the device may comprise: a patterned half wave retarder comprising first and second regions whose optic axes are oriented at +22.5xc2x0 and xe2x88x9222.5xc2x0 to the transmission axis of the polariser; and an unpatterned retarder whose optic axis is oriented at 67.5xc2x0 to the transmission axis of the polariser.
The illumination system may comprise a linear to circular polarisation converter, the device may comprise a quarter waveplate and a linear polariser, and the projection optics may comprise a circular to linear polarisation converter. Each of the linear to circular and circular to linear polarisation converters may comprise a linear polariser and a quarter waveplate.
The term xe2x80x9coptical powerxe2x80x9d as used herein has the conventional meaning, for example as disclosed in xe2x80x9cGeometrical Optics and Lens Designxe2x80x9d, P. Mouroulis and J McDonald, Oxford University Press, 1997, ISBN 0-19- 508931-6, pages 40 to 43. In the case of a single spherical surface, optical power K is defined by K=c(nxe2x80x2-n), where c is the curvature of the surface, n and nxe2x80x2 represent the refractive indices before and after the interface. For reflector systems, then nxe2x80x2=nxe2x88x92n. A surface must have a non-zero K value to possess optical power. A plane mirror has no power (c=0).
The focal length f of a single spherical surface is defined as 1/K. However, the focal length of a surface can be defined more generally and measured as the distance from the surface to the xe2x80x9ccircle of least confusionxe2x80x9d for a collimated illumination at the appropriate cone angle. Thus, for xe2x80x9cnon-perfectxe2x80x9dsurfaces, there is still an imaging property of the surface that can be measured. The focal length is defined as being the focal length when measured in the relevant medium, for example in a counter substrate of the spatial light modulator.
The circle of least confusion is defined, for example in xe2x80x9cOpticsxe2x80x9d, Hecht and Zajac, Addison-Wesley, 1974, page 176, as being at the position at which the image blur of the focused spot has its smallest diameter.
In the case of spherical or similar reflectors, there is optical power in two orthogonal directions which are perpendicular to the optical axis of the reflector. However, a reflector can have optical power in a single direction, for example when the reflector is of the cylindrically converging type. In this case, the circle of least confusion is replaced by a strip of least confusion and is at the position at which the image blur of the focused strip has its smallest width. In this case, the optical power K is defined in the same way as for a single spherical surface but is in one direction (in the case of a cylindrical reflector, perpendicular to the axis of the cylinder and to the optical axis of the reflector).
The focal length of the converging reflectors of the present optical device may be substantially equal to half the optical separation of the microlenses from the reflectors when the distance from an xe2x80x9cinputxe2x80x9d microlens for incident light to a reflector and the distance from the reflector to an xe2x80x9coutputxe2x80x9d microlens for the reflected light are the same i.e. the input and output light paths are symmetrical. In some embodiments where input and output pupils are displaced in the second direction, the distance between the microlens and the reflector includes the inclination of light beams through the spatial light modulator, for example through the thickness of glass of a counter substrate. In the case of non-symmetrical input and output paths, the focal length can be adjusted accordingly to allow the input aperture to be imaged onto the output aperture.
The optimum converging reflecting surface may be as aspheric surface or a spheric surface. Height variations of the surface may be made compatible with, for example, a liquid crystal device manufacturing environment. For example, the surface of each reflector may be faceted to minimise surface undulations for liquid crystal materials. Such a surface may be fabricated from a single layer of photosensitive material by a grey-scale masking technique.
In order to achieve the correct focal length, the tilt angle of the surface may vary across the width of a sub-aperture and the tilt angle of the surface at any part of the surface of the sub-pixel may be close to the tilt angle of the equivalent optimum surface. Thus, the reflecting surface is a sub-pixel cannot be a single plane surface.
Although a diffractive surface may not have a surface curvature, nevertheless a diffractive surface may be arranged to focus light with a focal length as defined above. Such diffractive surfaces or structures thus constitute reflectors having convergent optical power.
A plane surface has an infinite focal length whereas an assembly of faceted plane surfaces with appropriate variations in tilt angles has a measurable focal length. Accordingly, such a faceted surface also constitutes a reflector having convergent optical power.
Although the device according to the first aspect of the invention may have other applications and uses, for example in optical computing, this device is particularly suitable for use in a projection display in accordance with the second aspect of the invention.
It is thus possible to provide a projection system using a relatively large spatially multiplexed device in a relatively compact reflector arrangement. Although colour filtering may be provided within the device in order to provide a colour projection display, such absorptive colour filters can be omitted so as to avoid light loss due to absorptive filtering. High aperture ratio field reflectors can be used so as to maximise efficiency and may be manufactured relatively easily. For example, such reflectors may be manufactured using a metallised surface relief photoresist technique which is compatible with existing LCD fabrication techniques.
It is possible to provide an arrangement in which the final pixel appearance is white with a very high aperture ratio. Such arrangements are extendable to higher resolutions. Arrangements using the crossed polariser mode, for example with a single large area polariser and patterned retarder or with an input polariser in a condensing system of the illumination system and a xe2x80x9ccrossedxe2x80x9d output polariser at a point near a projection lens of the projection optics, have low light absorption and can provide a higher contrast ratio than may be achieved with standard single polariser mode arrangement. Also, front reflection artefacts from the device may be cancelled by means of a suitable polarisation arrangement before entering the projection optics and this provides an improved contrast ratio.
The device may make use of conventional liquid crystal modes so that it is possible to provide a low cost single panel projection system. For instance, high volume direct view display panels, possibly with relatively small modifications, may be used and specialist projection light valves are not required.
Vignetting which occurs in known types of displays can be eliminated or substantially reduced so as to reduce light loss and improve the display brightness for a given level of illumination. Also, input and output pupils can be separated so that a polarising beam splitter is not necessary. This reduces the cost and bulk of a projection display and eliminates the light losses associated with such beam splitters.
The use of a reflection panel allows a higher aperture ratio and hence the use of longer focal length microlenses compared with high resolution transmission type panels for reasons of efficient coupling of beam etendue through the system. Thus, it is more practical to incorporate the microlenses externally of the LCD and this reduces the cost and complexity of the system.
Some embodiment make use of a one dimensional microlens array, for example of the cylindrically converging lenticular screen type, and this has advantages over arrangements which use two dimensional microlens arrays. For example, a stripe LCD panel may be used rather than a delta LCD panel. Stripe panels are more easily available and are easier to manufacture then delta panel and non-standard pixel configuration panels.
Cylindrically converging lenses are easier to tolerance with respect to alignment with a stripe LCD panel. Also, such lenses are easy to make with substantially 100% aperture ratio compared with the generally lower aperture ratio of two dimensional microlenses. Further, such lenses have a reduced boundary area and this minimises scatter losses.
With such one dimensional microlens arrays, the output image is in the form of white stripes. This produces an image with substantially square pixels which provides improved viewed quality. In some embodiments, the input and output microlenses are different parts of the same lenticule so that irregularities between input and output surfaces may be minimised.
In the case where input and output pupils are separated along the cylindrical axis of the lenticules, the visibility of facct errors in faceted reflectors is reduced.
The reflecting element is a less complex shape and can be made to less strict tolerance compared with two dimensional microlenses because the imaging reflection is required in only one dimension. Thus, increased errors in surface shape can be accepted and this reduces complexity and cost. Also, there are fewer facet edges and this reduces light loss caused by poor imaging performance of facets. This also reduces the probability of degradation of alignment of the liquid crystal layer.